233 research outputs found
Decay of Hypernuclei
We present a nonrelativistic transition potential for the weak
strangeness-changing reaction . The potential is based on a
one meson exchange model (OME), where, in addition to the long-ranged pion, the
exchange of the pseudoscalar , as well as the vector mesons is considered. Results obtained for different hypernuclear decay
observables are compared to the available experimental data.Comment: 8 pages. Invited talk given at the KEK-Tanashi International
Symposium on Physics of Hadrons and Nuclei. Tokyo, Japan, December 14-17,
1998. In honor of Prof. K. Yazaki. Submitted to Nucl. Phys. A. LateX file
(uses espcrc1.sty
The Weak Decay of Hypernuclei
The nonmesonic weak decay of hypernuclei is studied in a shell
model framework. A complete strangeness-changing weak
transition potential, based on one boson exchange, is constructed by including
the exchange of the pseudoscalar mesons , K, as well as the vector
mesons , and K, whose weak coupling constants are obtained
from soft meson theorems and SU(6). General expressions for nucleons in
arbitrary shells are obtained. The transition matrix elements include realistic
N short-range correlations and NN final state interactions based on
the Nijmegen baryon-baryon potential. The decay rates are found to be
especially sensitive to the inclusion of the strange mesons, K and K, even
though the role of kaon exchange is found to be reduced with recent couplings
obtained from next-to-leading order Chiral Perturbation Theory. With the weak
couplings used in this study the rates remain dominated by the pion-exchange
mechanism since the contributions of heavier mesons either cancel each other or
are suppressed by form factors and short-range correlations. The total decay
rate therefore remains in agreement with present measurements. However, the
partial rates which are even more sensitive to the inclusion of heavier mesons
cannot be reconciled with the data. The proton asymmetry changes by 50% once
heavier mesons are included and agrees with the available data.Comment: 70 pages, 8 figures, epsf.tex, revtex, submitted to Phys. Rev.
Microscopic approach to the proton asymmetry in the non-mesonic weak decay of Lambda-hypernuclei
The non--mesonic weak decay of polarized -hypernuclei is studied
with a microscopic diagrammatic formalism in which one- and two-nucleon induced
decay mechanisms, and , are
considered together with (and on the same ground of) nucleon final state
interactions. We adopt a nuclear matter formalism extended to finite nuclei via
the local density approximation. Our approach adopts different
one-meson-exchange weak transition potentials, while the strong interaction
effects are accounted for by a Bonn nucleon-nucleon interaction. We also
consider the two-pion-exchange effect in the weak transition potential. Both
the two-nucleon induced decay mechanism and the final state interactions reduce
the magnitude of the asymmetry. The quantum interference terms considered in
the present microscopic approach give rise to an opposite behavior of the
asymmetry with increasing energy cuts to that observed in models describing the
nucleon final state interactions semi-classically via the intranuclear cascade
code. Our results for the asymmetry parameter in C obtained
with different potential models are consistent with the asymmetry measured at
KEK
Non-Mesonic Weak Decay of Lambda-hypernuclei: a new determination of the Gn/Gp ratio
Theoretical descriptions of the non-mesonic weak decay of Lambda-hypernuclei
are unable to reproduce the experimental values of the ratio Gn/Gp=G(Lambda n
-> nn)/G(Lambda p -> np). In this contribution we discuss a new approach to
this problem. We have incorporated a one-meson-exchange model for the Lambda N
-> nN transition in finite nuclei in an intranuclear cascade code for the
calculation of double-coincidence nucleon distributions corresponding to the
non-mesonic decay of 5_Lambda-He and 12_Lambda-C. The two-nucleon induced decay
mechanism, Lambda np -> nnp, has been taken into account within a local density
approximation scheme using a one-pion-exchange model supplemented by short
range correlations. A weak decay model independent analysis of preliminary KEK
coincidence data for 5_Lambda-He allows us to extract Gn/Gp=0.39 \pm 0.11 when
the two-nucleon induced channel is neglected (i.e., G2=0) and Gn/Gp=0.26 \pm
0.11 when G2/G1=0.2.Comment: 7 pages, 1 figure. Invited talk at the International Conference on
Hypernuclear and Strange Particle Physics (HYP2003), Jefferson Lab, Newport
News, Virginia (USA), Oct 14-18, 200
Functional approach to the non-mesonic decay of Lambda-hypernuclei
We present an evaluation of the non-mesonic decay widths for
Lambda-hypernuclei (Lambda N --> NN, Lambda NN --> NNN) within the framework of
the polarization propagator method. The full Lambda self-energy is evaluated
microscopically in nuclear matter by using the functional approach, which
supplies a theoretically well grounded approximation scheme for the
classification of the relevant diagrams, according to the prescriptions of the
bosonic loop expansion. We employ average Fermi momenta, suitably adapted to
different mass number regions (medium-light, medium and heavy hypernuclei).
Moreover, we study the dependence of the decay rates on the NN and Lambda-N
short range correlations. With a proper choice of the parameters which control
these correlations in the new approximation scheme, it is possible to reproduce
the experimental decay widths for A > 10 hypernuclei.Comment: 25 pages, 8 figure
Final State Interactions in Hypernuclear Decay
We present an update of the One-Meson-Exchange (OME) results for the weak
decay of s- and p-shell hypernuclei (Ref. Phys. Rev. C {\bf 56}, 339 (1997)),
paying special attention to the role played by final state interactions between
the emitted nucleons. The present study also corrects for a mistake in the
inclusion of the and exchange mechanisms, which substantially
increases the ratio of neutron-induced to proton-induced transitions,
. With the most up-to-date model ingredients, we find that
the OME approach is able to describe very satisfactorily most of the measured
observables, including the ratio .Comment: 20 pages, 2 eps figure
Towards the solution of the Gn/Gp Puzzle in the Non-Mesonic Weak Decay of Lambda-Hypernuclei
One of the main open problems in the physics of Lambda-hypernuclei is the
lack of a sound theoretical interpretation of the large experimental values for
the ratio Gn/Gp=G(Lambda n -> nn)/G(Lambda p -> np). To approach the problem,
we have incorporated a one-meson-exchange model for the Lambda N -> nN
transition in finite nuclei in an intranuclear cascade code for the calculation
of single and double-coincidence nucleon distributions corresponding to the
non-mesonic weak decay of 5_Lambda-He and 12_Lambda-C. Due to the elimination
of interferences, two-nucleon coincidences are expected to give a cleaner
determination of Gn/Gp than single-nucleon observables. Single-nucleon
distributions are found to be less sensitive to variations of Gn/Gp than
double-coincidence spectra. The comparison of our results with preliminary KEK
coincidence data allows us to extract a Gn/Gp ratio for 5_Lambda-He of
0.39+-0.11 when multinucleon induced channels are omitted.Comment: 12 RevTeX pages, 12 figure
An EFT for the weak interaction
The nonleptonic weak interaction, responsible for
the dominant, nonmesonic decay of all but the lightest hypernuclei, is studied
in the framework of an effective field theory. The long-range physics is
described through tree-level exchange of the SU(3) Goldstone bosons ( and
), while the short-range potential is parametrized in terms of lowest-order
contact terms obtained from the most general non-derivative local four-fermion
interaction. Fitting to available weak hypernuclear decay rates for , and yields
reasonable values for the low-energy constants.Comment: Contribution to the Proceedings of the VIII International Conference
on Hypernuclear and Strange Particle Physics, HYP03, revised version. 10
pages. Uses espcrc1.st
Single and double coincidence nucleon spectra in the weak decay of Lambda hypernuclei
Recent progress has been experienced in the field of hypernuclear weak decay,
especially concerning the ratio of the neutron- to proton-induced Lambda
non-mesonic decay rates, G_n/G_p. Theoretical analyses of nucleon coincidence
data have been performed in a finite nucleus framework. They led to the
extraction of G_n/G_p values in agreement with pure theoretical estimates, thus
providing an evidence for the solution of a longstanding puzzle. Here we
present an alternative approach to the problem, based on a nuclear matter
formalism extended to finite nuclei via the local density approximation. The
work is motivated by the exigence to make the determination of G_n/G_p from
data less model dependent. One-meson-exchange potentials are used for
describing both the one- and two-nucleon induced decays, Lambda N -> n N and
Lambda N N -> n N N. For the latter, treated within a microscopic approach, the
channels Lambda n n -> n n n and Lambda p p -> n p p are included in addition
to the mode Lambda n p -> n n p already considered, in a phenomenological way,
in previous studies. The propagation of the final nucleons in the residual
nucleus is simulated by an intranuclear cascade code. We evaluate single and
double coincidence nucleon spectra for the non-mesonic decay of C-12-Lambda.
Through the comparison of our predictions with KEK coincidence data we
determine G_n/G_p=0.43 \pm 0.10 for this hypernucleus, confirming previous
finite nucleus analyses.Comment: 34 pages, 17 figure
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